Yes, there is! The term you are looking for is continuation novel.
Wikipedia defines it:
A continuation novel is a novel in the style of an established series, produced by a new author after the original author's death
Continuation novels may be official, produced with the permission of the late author's literary executors, or unofficial where the original author's works are now out of copyright.
History Today wrote on "Picking Up the Torch: The Golden Age of the Continuation Novel" in Mach 2015:
The concept of one novelist ‘writing as’ another, usually to expand or continue a literary franchise, is known as a continuation novel, and is an increasingly familiar feature of the contemporary literary landscape. ...
The continuation novel differs from fan fiction (also enjoying a purple patch, which is unlikely to be a coincidence) chiefly in its ‘official’ nature. The books are commissioned by the deceased author’s estate, written with its approval, and marketed using both author’s brand associations. They are, in a sense, the logical conclusion of the familiar marketing tool of comparing a new author to an established author: ‘For fans of…’ or ‘Author A meets Author B’. The first well-known example of this model of publishing with an established author ‘picking up the torch’ on official business does not occur until the post-war period, when, in 1968, Kingsley Amis wrote Colonel Sun, a ‘new’ James Bond novel under the pseudonym Robert Markham at the behest of Ian Fleming’s estate. Tantalisingly one reviewer of Amis’ book noted that while it was unusual for an established author to continue a franchise in this manner, it was not unprecedented, though my search for an example predating 1968 has proved fruitless thus far. Continuation novels have detractors. ‘The posthumous pseudo-sequel never amounts to more than a nostalgic curiosity’ wrote, ironically, Kingsley’s son Martin Amis in a review of Perchance to Dream, Robert B. Parker’s authorised attempt to resurrect Chandler in 1991.